1. Field of the Invention
The present invention relates to an air flow measuring device and to a technology appropriately used for measuring the amount of air flow (intake air amount) suctioned to an engine (internal combustion which generates rotative power from burning of fuel).
2. Description of Related Art
First to third conventional technologies of an air flow measuring device will be described by using sectional views of a main feature illustrated in FIGS. 5 to 7C.
The first conventional technology will be described bellow. An air flow measuring device 101 illustrated in FIG. 5 includes a passage forming housing 103 that defines a measured passage (e.g., a tertiary air passage) 102 in which measured air (measuring object) flows, a sensor housing 105 attached to a sensor attaching part 104 which is formed on the passage forming housing 103, and a sensor part 106 which is disposed on an end of the sensor housing 105 and measures a flow rate of the measured air flowing through the measured passage 102 (see, e.g., Japanese Patent No. 4358517).
The sensor housing 105 is attached to the sensor attaching part 104 of the passage forming housing 103. Therefore, as shown in FIG. 5, it has possibility that a “gap X” is formed between the passage forming housing 103 and the sensor housing 105 because of dimensional variation of components and other reason.
In this way, if the “gap X” is formed between the passage forming housing 103 and the sensor housing 105, turbulence of air flow toward the sensor part 106 (see a devious arrow in FIG. 5) is generated, and causes variation of output from the sensor part 106. Depending on the shape (curve) of the measured air on the upstream side of the sensor part 106 in the measured air flow direction, influence of the “gap X” increases and the variation of the output from the sensor part 106 becomes great.
The second conventional technology will be described bellow. As shown in FIGS. 6A to 6C, a part of a sensor housing 205 (hereinafter referred to as “an inner wall forming part 205a”) sometimes serves as a part of an inner wall of a measured passage 202. In this case as well, as shown in FIG. 6A, if the “gap X” is made between a passage forming housing 203 and the sensor housing 205, turbulence of air flow toward a sensor part 206 is generated (see a devious arrow in FIG. 6A) and causes variation of output from the sensor part 206.
In the case where the inner wall forming part 205a is formed on the sensor housing 205, it is difficult to make “the inner wall of the inner wall forming part 205a (the inner wall of the sensor housing 205)” coplanar with “an inner wall of the passage forming housing 203” with a high degree of accuracy. For this reason, as shown in FIG. 6B, “the inner wall forming part 205a of the sensor housing 205” may project into the measured passage 202, and a “projecting step difference Y” due to the inner wall forming part 205a is possibly formed in the measured passage 202. Alternatively, as shown in FIG. 6C, “the inner wall forming part 205a of the sensor housing 205” may be recessed from the inner wall of the measured passage 202, and a “recessed step difference Z” due to the inner wall forming part 205a is possibly formed in the measured passage 202.
In this way, if the “projecting step difference Y” or the “recessed step difference Z” due to the inner wall forming part 205a is made in the measured passage 202, the turbulence of the air flow toward the sensor part 206 is generated (see devious arrows in FIGS. 6B and 6C) and causes the variation of the output from the sensor part 206. Moreover, depending on the shape (curve) of the measured air on an upstream side of the sensor part 206 in a measured air flow direction, influence of the “projecting step difference Y” or the “recessed step difference Z” increases, and the variation of the output from the sensor part 206 becomes great.
The third conventional technology will be described below. In the above, the first and second conventional technologies are described by using FIGS. 5 to 6C in which the sensor housing 105 and 205 are disposed at a curved part α′ of the measured passage 102 and 202 respectively. Also in the case where a sensor housing 305 is disposed at a straight part β′ of a measured passage 302 as shown in FIGS. 7A to 7C, the “gap X”, the “projecting step difference Y”, or the “recessed step difference Z” described above causes a similar defect (turbulence of air flow toward a sensor part 306).